Potassium

Potassium is the seventh most abundant element in the Earth's crust and one of the most vital minerals for life. It controls the heartbeat, fires the nerve impulses that carry every thought and sensation, and keeps your muscles working. Without enough potassium, your heart would stop.

  • Atomic Number1919 protons, 19 electrons
  • Atomic Mass39.0983 uAbout 39× heavier than hydrogen
  • State at Room TempSolidsoft, silvery metal
  • Density0.89 g/cm³Less dense than water
  • Melting / Boiling63.4°C / 758.9°CMelts at just 63°C
  • Discovered1807Humphry Davy, 1807

How does potassium compare to other alkali metals?

Potassium sits below sodium in Group 1. Each alkali metal is heavier and more reactive going down the group.

Atomic Mass Comparison
Lithium6.9 u
Sodium23 u
Potassium39.1 u
Iron55.8 u
Rubidium85.5 u

Potassium (39 u) is approx. 70% heavier than sodium (23 u) and reacts even more vigorously with water, bursting into a violet flame instead of sodium's yellow one. Despite this reactivity, both are essential nutrients your body balances carefully.

What is potassium?

Potassium is an alkali metal in Group 1 of the periodic table, one step below sodium. It has 19 protons and a single electron in its outer shell, which it gives away so eagerly that it reacts explosively with water. Pure potassium is a soft, silvery metal that can be cut with a table knife. Like sodium, it must be stored under oil to prevent it reacting with air and moisture.

The English name potassium comes from potash, an old name for potassium carbonate (K₂CO₃), made by soaking wood ash in water in a pot. The chemical symbol K comes from the Latin kalium, itself from the Arabic al-qali (the alkali), referring to the same wood-ash material. Both names ultimately trace back to the same ancient material used for thousands of years to make soap and glass.

Fact Potassium reacts with water more violently than sodium. A small piece dropped into water bursts immediately into a brilliant violet-lilac flame as the hydrogen gas produced ignites. Larger pieces can explode. Yet the potassium in your body, approx. 140 grams of it, is essential for every heartbeat and every thought you have.

Where you find potassium

In space

Potassium is found in stars and throughout the universe. On Earth, the radioactive isotope potassium-40 has provided a major source of internal heat for the planet since its formation, a slow decay that has helped drive plate tectonics and volcanism for billions of years.

On Earth

Potassium is widespread in the Earth's crust, found in many minerals and dissolved in the sea.

  • Sylvite and carnallite. These potassium chloride minerals are the main ores mined for potassium. Canada, Russia, Belarus and Germany are among the world's largest potash producers.
  • Feldspar and mica. Many common rock-forming minerals contain potassium. As these rocks weather, potassium is released and taken up by plants.
  • Living things. Potassium is the most abundant positively charged ion inside cells. Your body contains approx. 140 grams of potassium, mostly inside muscle and nerve cells.

How we use potassium

  • Fertilisers. About 95% of the potassium mined worldwide goes into fertilisers. It is one of the three essential plant nutrients (with nitrogen and phosphorus), strengthening stems, improving drought resistance and promoting flowering and fruiting.
  • Food and nutrition. Potassium chloride is used as a reduced-sodium salt substitute. Bananas, potatoes and leafy vegetables are all rich sources of dietary potassium.
  • Soap making. Potassium hydroxide (caustic potash) is used to make soft soaps and liquid soaps. Sodium hydroxide gives hard bar soap; potassium hydroxide gives liquid soap.
  • Medicine. Potassium chloride injections treat dangerous potassium deficiency. Potassium compounds are also used in treating certain heart conditions.
Did you know? You have approx. 140 grams of potassium in your body right now, and a tiny fraction of it is radioactive. About 0.012% of all natural potassium is potassium-40, a radioactive isotope that slowly decays. This means your body emits approx. 4,000 radioactive decay events per second, mostly from potassium-40 in your muscles.

How it was discovered

Potassium was isolated on the same famous evening as sodium in 1807, by Humphry Davy at the Royal Institution in London. He passed an electric current through molten potassium hydroxide and small globes of bright silvery metal appeared at the electrode. When they touched the air, they immediately burst into a brilliant violet-pink flame. Davy reportedly danced around the room in excitement. It was the first time any metal had been obtained by electrolysis, a technique that would go on to unlock many other elements.

Deeper dive: potassium in biology: nerve impulses and the sodium-potassium pump

Potassium is the main positively charged ion (cation) inside cells, while sodium is the main cation outside. This imbalance, maintained at great energy cost by every cell in your body, creates the electrical potential across the cell membrane that makes nerves and muscles work.

The sodium-potassium ATPase pump sits in cell membranes and uses energy from ATP to push three sodium ions out and pull two potassium ions in for every cycle. This creates and maintains the concentration gradient. When a nerve fires, sodium channels snap open and sodium floods in, rapidly reversing the charge across the membrane. Potassium channels then open and potassium flows out, restoring the original charge. This travelling wave of charge reversal is a nerve impulse, an action potential, carrying information at up to 120 metres per second.

Low blood potassium (hypokalaemia) causes muscle weakness, cramps and dangerous heart arrhythmias. High potassium (hyperkalaemia) can stop the heart. The kidneys regulate blood potassium levels with remarkable precision, adjusting how much is excreted in urine. Diuretic medicines used for high blood pressure can deplete potassium, which is why doctors sometimes prescribe potassium supplements alongside them.

Potassium is one of the great elements of life, every heartbeat depends on it. Moving one step to the right on the same period brings us to calcium, the mineral that builds your bones and teeth.